With the increasing EV (electric vehicle) design of automobiles, the consumption of harnesses and connectors that are connection parts of wires tends to increase. In the adoption of EVs, ensuring safety and gas mileage by electronic control techniques is also a purpose.
Connectors that are incorporated in automobiles are used in severe environments of high temperature and vibration and, therefore, the reliability of connection and contact stability are required. Also, with increasing adoption of EVs, copper-based materials that have small energy losses, i.e., high conductivity are desired.
Also for electrode materials for welding, properties having values above prescribed ones are required in all respects of mechanical strength, thermal properties and electrical properties.
For mechanical strength, it is known as the Hall-Petch law that mechanical strength is generally improved by refining the crystal structures of metal materials.
For example, when metal or alloy materials are deformed, material strength increases due to work hardening. This is understood as follows. That is, various kinds of defects (point defect, dislocation, stacking fault, etc.) are accumulated in materials due to working (plastic deformation), and as a result of the interactions of these defects, the introduction and migration of new defects become difficult and the materials obtain resistance to external force.
To apply plastic deformation (strain) to metal materials, extrusion, drawing, shearing, rolling, forging, etc. have hitherto been carried out. Concretely, the HIP (High Pressure Torsion) process that involves twisting a material while applying high pressure to the material, the CEC (Cyclic Extrusion Compression) process that involves repeatedly threading a material through a constricted pipe, and the ARB (Accumulative Roll Bonding) process that involves cutting a metal sheet the thickness of which has been reduced by rolling and repeatedly rolling superimposed metal sheets have been proposed, and in particular, as a concrete method of refining the grains of an aluminum alloy, the ECAE (equal-channel-angular extrusion) process that involves applying shearing deformation to a material by lateral extrusion without a reduction of sectional area of the material has been proposed as disclosed in the Japanese Patent Laid-Open No. 9-137244, the Japanese Patent Laid-Open No. 10-258334, the Japanese Patent Laid-Open No. 11-114618, the Japanese Patent Laid-Open No. 2000-271621, etc.
On the other hand, for copper alloys, methods disclosed in the Japanese Patent Laid-Open No. 11-140568, the Japanese Patent Laid-Open No. 2000-355746, etc. have been proposed. In these conventional techniques, to improve the properties (machinability and dezincification corrosion) of brass (Cu—Zn) that is used as a material for water faucet fittings and the like among other copper alloys, dynamic recrystallization is caused to occur by hot extrusion thereby to obtain the refinement of crystal grains and specific ratios of crystal structures (ratios of the α-phase, β-phase and γ-phase).
Also, to bring out prescribed properties from age-hardening type copper alloys to which an element that does not dissolve or scarcely dissolves in a solid solution state at room temperature, such as chromium (Cr), zirconium (Zr), beryllium (Be), titanium (Ti) and boron (B), is added, this element is first caused to dissolve sufficiently in a solid solution sate at a high temperature and then quenched and brought to a supersaturated condition, which is followed by aging treatment at a prescribed temperature, thereby causing the added element in a supersaturated condition to precipitate.
Even when the above-described work aging or aging treatment for aluminum alloys and copper alloys is applied as it is to age-hardening type copper alloys to which an element, such as chromium (Cr), zirconium (Zr), beryllium (Be), titanium (Ti) and boron (B), is added, it is impossible to simultaneously satisfy all respects of mechanical strength, thermal properties and electrical properties.
That is, in order to ensure that the thermal properties and electrical properties required of connectors used in electric vehicles or the like, electrode materials, etc. are developed, it is necessary to ensure that an added element that dissolves in a solid solution state is caused to precipitate in the largest possible amount. In order to cause this element to precipitate in a large amount, it is necessary to raise the aging temperature. However, when the aging temperature is raised, grain growth proceeds and mechanical properties decrease. That is, mechanical strength and thermal and electrical properties are in a tradeoff relation.
For thermal properties and electrical properties, copper alloys in which an oxide such as alumina is dispersed in the copper matrix are excellent in electrical conductivity and heat resistance and, therefore, these copper alloys are widely used in materials for electric parts. Many proposals to improve the properties and manufacturing methods of these copper alloys have been made.
For example, a proposal has been made to improve electrical conductivity and softening properties by adding, as elements that perform internal oxidation, not only aluminum, but also tin as a third element. (Japanese Patent Laid-Open No. 59-150043)
There has been proposed a copper alloy in which the amount of particles of not more than 50 μm is not less than 70 wt % owing to the use of a copper alloy powder of not more than 300 μm which is manufactured by the atomizing process and in which a readily oxidizing metal such as aluminum is caused to dissolve in a solid solution state. (Japanese Patent Laid-Open No. 60-141802)
There has also been proposed a method that involves internally oxidizing a Cu—Al alloy powder thereby to convert Al to Al2O3, making the surface of this alloy powder smooth, green compacting the powder to form a green compact, and hot forging this green compact at 600 to 1,000° C. (Japanese Patent Laid-Open No. 63-241126)
Also, there has been proposed a method that involves internally oxidizing a plate-like copper alloy containing Al to convert Al to Al2O3, working this plate-like alloy in coil form, sealing this coiled alloy in a metal tube, and hot working this metal tube at 900° C. in a desired shape. (Japanese Patent Laid-Open No. 2-38541)
Also, there has been proposed a method that involves filling an alloy powder obtained by internally oxidizing Cu—Al alloy chips in a carbon die and hot pressing the alloy powder at 900° C. and at a pressure of 400 kg/cm2. (Japanese Patent Laid-Open No. 2-93029)
Furthermore, there has been proposed a method that involves improving sinterability by causing an annular hard layer of Al2O3 to be present in the interior of a Cu—Al alloy powder. (Japanese Patent Laid-Open No. 4-80301)
In all of the above-described conventional techniques, hot working at high temperatures is performed and, therefore, structures tend to become coarse due to grain growth. Thus, in the conventional methods, it is impossible to obtain materials that simultaneously satisfy, as the properties required of connectors of electric vehicles and electrode materials for welding, the requirements that hardness be not less than 30 HRB, preferably not less than 40 HRB, that electrical conductivity be not less than 85 IACS %, preferably not less than 90 IACS %, and that thermal conductivity be not less than 350 W/(m·K), preferably not less than 360 W/(m·K).
When hardness is not less than 30 HRB, it is possible to prevent the tip of an electrode material from becoming deformed and generating heat. When electrical conductivity is not less than 85 IACS %, it is possible to prevent an electrode material from reacting with a steel sheet and sticking to the steel sheet. When thermal conductivity is not less than 350 W/(m·K), it is possible to prevent the deposition of an electrode material during welding because the cooling efficiency increases.
Because Al2O3 does not dissolve in Cu in a solid solution state even at a high temperature, a conventional technique by which Al2O3 is caused to precipitate by aging treatment after dissolution in a solid solution cannot be applied to a Cu—Al alloy.